1. Field of the Invention
The present invention relates generally to apparatus for storing control rod drive shafts during chemical decontamination of a reactor vessel and, more particularly, to such apparatus for removing the shafts from the reactor vessel and placing them in a storage tank while maintaining the shafts completely submerged in borated water.
2. Background of the Invention
In nuclear power generation, a reactor vessel is the primary vessel wherein heat is generated for producing steam. The reactor vessel includes a flanged body having a flanged, removable upper closure head bolted atop its upper portion for forming a sealed enclosure. Fuel pellets, which are located within fuel assemblies, are positioned within the reactor vessel for producing a controlled nuclear fission which, in turn, generates the necessary heat. Process piping, generally referred to in the art as a primary loop, is attached to the reactor vessel for passing borated water contained in the reactor vessel therethrough. Borated water is contained within the reactor vessel for conveying the generated heat away the reactor vessel. The borated water flows out of the reactor vessel and passes through the primary loop to a steam generator for transferring its heat to a secondary loop, wherein steam is produced for ultimately producing electrical power, as is well known in the art. The borated water then returns to the reactor vessel via the primary loop where the above described process is repeated.
To control the nuclear fission process, a plurality of control rods are either selectively inserted or withdrawn from the fuel assemblies. Control rods are typically stainless steel tubes encapsulating an absorber material, and are grouped together in a predetermined number, generally sixteen, forming a control rod cluster. The control rod clusters extend into the fuel assemblies when fully inserted, and when the control rod clusters are withdrawn, they extend up and away from the fuel assemblies.
A control rod drive shaft is attached to each cluster and extends upwardly away from the fuel assemblies where each shaft is attached to a control rod drive mechanism (CRDM) for axially moving the control rod shafts and, in turn, the absorber material within the stainless steel tubes. An electromagnetic coil stack assembly is attached to the CRDM for electromagnetically supplying the CRDM the energy necessary to move the control rod clusters.
During reactor operation, corrosion creating products such as ions are found throughout the primary loop. These products circulate with the borated water through the reactor, where they may become irradiated or activated by the nuclear process occurring therein. The activated ions then deposit on the various surfaces within the reactor vessel which, in turn, creates an oxide layer on these surfaces. The oxide layer emits radiation which during maintenance presents drawbacks. One such drawback is that nuclear regulations regulate the amount of radiation exposure to workers, and to stay well within these limits during maintenance, the radiation emitted by the oxide layer should be minimized.
Although reductions in personnel exposure to radiation during maintenance have been achieved through the industry's aggressive radiation management programs; obviously, improvements in such a dynamic technological field are always needed. To thoroughly alleviate this radiation exposure problem (i.e., the activated oxide layer), the oxide layer should be removed from the reactor. One method of significantly reducing this source of radiation is to chemically decontaminate the entire primary loop.
To accomplish decontamination, the reactor is deactivated, and a reagent, such as the CAN-DEREM (developed oped by Atomic Energy of Canada, Ltd.) or LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board), is passed through the reactor vessel and the primary loop for performing decontamination. Both CAN-DEREM and LOMI are well known in the art for decontamination. The process of decontamination is disclosed in U.S. Pat. No. 5,132,076 which is assigned to the assignee of the present invention.
Although the presently known and utilized method and device for decontamination are satisfactory, they are not without drawbacks. Studies have shown that the reagents used during the decontamination process may cause structural weakening of some components, more particularly the control rod drive shaft. Therefore, the control rod drive shafts should be removed to minimize any such mechanical weakening. In addition, because the control rods have been exposed to radiation, they should, at all times during such removal and storage, be placed in a medium which minimizes radiation exposure to the personnel performing these tasks. Presently, there are no known devices or methods for removing and storing the shafts during decontamination while keeping them in a medium which reduces radiation exposure to the personnel.
Consequently, a need exists for storing the control rod drive shafts in a medium during decontamination.